In-vitro antimicrobial susceptibility of Rhodococcus equi

Rhodococcus equi is an intracellular facultative, Gram-positive cocco-bacillary organism of increasing importance as a pulmonary pathogen in HIV-positive patients. This study was carried out to evaluate the optimal antibiotic combinations for treating such infections. Four human R. equi isolates and one reference strain were tested for their susceptibilities to 36 antibiotics. In-vitro the most active antibiotics were amikacin, gentamicin, netilmicin, erythromycin, clarithromycin, roxithromycin, ciprofloxacin, sparfloxacin, rifampicin, vancomycin, teicoplanin, doxycycline, minocycline, imipenem, meropenem and trimethoprim/sulphamethoxazole. The only bactericidal antibiotics were the aminoglycosides, ciprofloxacin, sparfloxacin and vancomycin. As determined by FIC indices, four combinations were synergistic: rifampicin-erythromycin, rifampicin-minocycline, erythromycin-minocycline and imipenem-amikacin. However, no antibiotic combinations were synergistic with the time-kill kinetic method at achievable serum concentrations or at ten-fold and half-fold the MICs. Frequencies of selection of antibiotic-resistant mutants determined at concentrations of five- and ten-fold the MICs ranged from less than 1 x 10(-8) for erythromycin and trimethoprim/sulphamethoxazole to 5 x 10(-4) for amikacin. These results may be of help in selecting the antibiotics for treating infected HIV-positive patients.